Max CN1 input Voltages (both + and - )

Products & Technology DSO Nano
#1

Update: 2/28/2011 Updated the below attachment to increase understanding (changed Ra to R2 and also changed Rb to Ru4 x7-x3).

I recently asked about the maximum negative input voltage allowed, but never got a response. Sooooo … I did my own research. Attached is a zip with supporting files.

This presentation is based upon the absolute maximum specifications of the U4 and U5 chips, and the 0-3V requirements of the ADC. These results are based upon the V1.3A hardware schematic, however, I noticed that the V2.3 hardware schematic shows a different positive supply voltage for U5. I suspect that is a typo.

The XLS file is set up so you can change the probe voltages, and all other changes will ripple through.

I would appreciate if others would validate my Excel spreadsheet formulas and equivalent front-end circuits for any possible amendments to make the results more accurate.

Update: 2/28/2011 Updated the below attachment to increase understanding (changed Ra to R2 and also changed Rb to Ru4 x7-x3).
front end3.zip (326 KB)

#2

Looking at your calculations, it would seem that the U5B gain is a bit off. This should be about 4.07 (calculated from 1k1 and 270). The 6k and 3k resistors in your spreadsheet are actually 6k8 and 3k3 according to the schematic (V1.1 and V2.3).

#3

Thanks for that feed-back. I am not familiar with the 6k3 label, I thought it was a multi-part indicator such as 3-ea 6K in one package. I will make those changes and post the new file with an “A” appended to the zip filename.

I searched this topic and discovered that this exponent encoding was adopted for small surface-mounted components. I have been out of the electronics industry for about 8-years, so I guess I just learned something new here. :wink:

Thanks, this is exactly why I wanted some review before I released the results in a stand-alone post. Still looking for any additional comments from others.

#4

I still think there is more to this. Your max CN1 voltages are based on not exceeding the +/- 5V supply to the op-amps. I see these as constraints that need to be observed, but the limiting factor is likely to be output from U5A. This output must stay between ground and the ADC reference voltage of 3V. U5A is used to bias the output, so the absolute input voltage is less of an issue, but the differential voltage swing is.

U5A and U5B form a multistage op-amp and I’m not sure how to calculate the individual and combined gain for this stage (note that U5A has a feedback to U5B output). Perhaps someone more intimate with electronics than me could help shed some light on this.

When I do the reverse calculation based on conversion factors supplied by Seeed (factory calibration), I get a combined gain factor of approximately 5.08. We could use this in calculations, but seeing these figures match up with a schematic analysis would be better.

#5

I agree with your concerns. I will do some more digging about inverting opamp gain with offset.

electronics-tutorials.ws/opamp/opamp_2.html Inverting gain = -(Rf/Rin) = -1

electronics-tutorials.ws/opamp/opamp_3.html non inverting gain = 1 + (Rf/R2) = 5.07

I am sure that the U5A is configured with a gain of unity (which is typical for the second stage to prevent bandwidth loss). R-19 (22R) is of a low value to keep the already low U5A impedence low as seen ny the ADC.

I had initially dismissed this offset as a fixed DC voltage. The offset voltage could be programmable via “V0” Cpu port PB11, also known as CPU pin-48. The firmware port initialization call this pin an Alternate output. Maybe you can provide each range value if this is a programmable offset voltage. Or maybe that pin is just set to a high static state. Because of the inversion, with V0 set to a high state, then all positive inputs to CN1 would fall between 0 and 3V; but no negative CN1 values would be processed.

Waiting to hear about this V0. :smiley:

#6

Perhaps this change in V2.3 schematic is not a typo but instead a fix to limit the input to U5A. This coupled with a 3V static pin @ V0 and unity gain for U5A would only pass 0-3V to the ADC for positive inputs within the range scales. :astonished:

I confirmed this by fixing the gains and running the numbers in the XLS chart. New attachment of the changes.

Latest edit: Not true, it would have little impact upon the ADC input signal, therefore it is probably a typo.

#7

I looked closer at this and see that the first op-amp is non-inverting and so the formula should be (1 + R1/R2). This results in a total gain of 5.07 which match the reverse calculations exactly.

Solving the equations for max probe voltage yields the following:

Probe	Scale	                Range Max	Probe Max

1x	10mV, 20mV, 50Mv, 100mV   0,8	          1,18
1x	200mV, 500mV, 1V          8               8,9
1x	2V, 5V, 10V              80              92
10x	0.2V, 0.5V, 1V	       10	           11,6
10x	2V, 5V, 10V	          80	           86
10x	20V, 50V, 100V	      800	          838

Sensitivity is as follows:

288,7uV
2,2mV
22,6mV
2,8mV
21,1mV
204,7mV

Leaving some margin for error, I would say range max is a good limit to observe and also easy to remember.

#8

We both posted within one minute of each other so I know what you have been doing. My post has similar conclusions except for the 500V limit that I specify.

#9

Not out of the woods yet! Look at my buffer capture shown in the attached. Look at data point 893. Negative numbers?

What do you think this means?
what_the.zip (63.1 KB)

#10

I see the point, but no issue. This is just telling me that the positive probe terminal has a lower voltage than the negative terminal.

Negative input is possible (if this is your concern) because the input to U5A is biased to fit within the ADC 3V range. With ground placed at vertical center (on the Nano that is), an equal range is available for negative and positive voltage swings (50% of max). If we move ground down, more of the range is available for positive voltage swings. The bias voltage can be set to anywhere from 0V to 3V and so we can shift the range from all negative to all positive and anywhere in between.

#11

Yes! Ground is the one variable that we hadn’t discussed previously. And it appears that changing GndPos changes the VO that feeds the offset to U5A. I cranked that concept into the spreadsheet and included all supporting files in the new attachment for the initial post. The PDF file shows a visual for the GndPos changes.

The GndPos can only be moved from 0% (bottom of screen) up to 72% (top) of the screen and that limits the negative inputs that are allowable.

It was a struggle to get here, but thanks to BenF’s help, my conclusions are shown below for the six ranges and probes.

Only while the Nano is powered >>> ON <<<, a good rule of thumb for safe measurements is to use the Nano (V/Div x 8) numbers for positive inputs and 70% of the Nano (V/Div x 8) numbers for negative input signals. This also assumes that you have positioned the GndPos properly before applying the external signal to the probe:

Absolute max +V Limits @ CN1 (probe jack) with zero tolerance components and Nano power ON.
1.15 @ 1xprobe 10mV, 20mV, 50mv, 100mV
8.9 @ 1xprobe 200mV, 500mV, 1V
90 @ 1xprobe 2V, 5V, 10V
1.15 @ 1xprobe 0.2V, 0.5V, 1V /// 11.5 @ 10xprobe
5.2 @ 1xprobe 2V, 5V, 10V /// 52 @ 10xprobe
50 @ 1xprobe 20V, 50V, 100V /// 500 @ 10xprobe

Absolute max -V Limits @ CN1 (probe jack) with zero tolerance components and Nano power ON.
-0.828 @ 1xprobe 10mV, 20mV, 50mv, 100mV
-6.408 @ 1xprobe 200mV, 500mV, 1V
-64.8 @ 1xprobe 2V, 5V, 10V
-0.828 @ 1xprobe 0.2V, 0.5V, 1V /// -8.28 @ 10xprobe
-3.744 @ 1xprobe 2V, 5V, 10V /// 37.44 @ 10xprobe
-36 @ 1xprobe 20V, 50V, 100V /// 360 @ 10xprobe

#12

You should be able to move Gnd Pos anywhere you like as long as you don’t push trigger level (+/- sensitivity) off the screen at the same time.

#13

Ben, I am unable to do this and would like to. Gnd Pos is firmly locked inside the visible screen. Am I missing something to enable it?

#14

“Anywhere” is a tad strong as it is limited by the boundaries of the waveform area. In the context of lygra’s post however, this is the 0% to 100% (as opposed to 72%) needed to use the full voltage range capability of the Nano.

#15

Ah right. I misunderstood you.

Thank you for clarifying that for me.

#16

I have provided an updated attachment in the original post of this thread. It includes error corrections and also provides data concerning input probe attenuation and that effect upon the Nano display results. This should provide a more complete picture of the Nano input circuits.

#17

i am totally confused …do you mean the X10 proble is just add a serial 9M resistor ? but I do not have a X10 proble …

#18

Most all 10X probes do in fact add 9M-ohm of resistance to the Nano input circuit. It also has a small amount of capacitance that is trim adjusted as probe compensation.

Without the 10x probe, then just use the first three rows (1x rows) of the spreadsheet. There is no advantage to using the Nano 10x settings with a 1x probe. This can be seen by looking at the CN1 input column of the spreadsheet. The maximum CN1 input values for 10x are similar to or less than the maximum CN1 input values for 1x settings.

You can click on most cells to observe the formula used for that cell’s results.